The typical energy conversation efficiency of today's commercially available renewable power generators is not much greater than 10%. Therefore, transferring the converted energy to the load in an efficient manner is of critical importance for producing economically viable renewable energy.
FIG. 1 illustrates a graph 100 depicting the electrical current versus voltage characteristics for a power generator and the resulting electrical power generated based upon the respective voltage and current supplied by the power generator.
For graph 100, an x-axis 102 represents the voltage supplied by the power generator in units of Volts, a first y-axis 104 represents the current supplied by the power generator in units of Amperes and a second y-axis 106 represents the power supplied by the power generator in units of Watts. Graph 100 includes an IV function 108 and a dashed power function 110. IV function 108 represents the current versus voltage characteristics of the power supply. IV function 108 includes a point 112, a point 114 and a point 116. Dashed power function 110 represents the electrical power generated by the power supply based upon the current versus voltage characteristics of the power supply. Dashed power function 110 includes a point 118, a point 120 and a point 122. Point 112 of IV function 108 represents a cutoff point where a value of zero voltage may be produced and thereby a zero quantity of power may be produced as represented by point 118 of dashed power function 110. Point 116 of IV function 108 represents a cutoff point where a value of zero current may be produced and thereby a zero quantity of power may be produced as represented by point 122 of dashed power function 110. Point 114 of IV function 108 represents a position of bend for IV function 108 and coincides on x-axis 102 with a maximum value for dashed power function 110 as denoted by point 120. Point 120 represents the Maximum Power Point (MPP) for the power generator and a dashed line 124 extends vertically downward from point 114 of IV function 108 to a point 126 on x-axis 102 and through point 120 of dashed power function 110. Point 126 represents the value of the voltage exhibited by the power generator at MPP. A dashed line 128 initiates at point 114 and extends horizontally to a point 130 of first y-axis 104. Point 130 represents the value of the current generated by the power generator at MPP. Dashed line 132 initiates at point 120 of dashed power function 110 and extends horizontally to a point 134 of second y-axis 106. Point 134 represents the value of the power generated by the power generator at MPP.
It may be observed that the power output of a renewable power generator peeks at a Maximum Power Point (MPP), denoted as point 120 on the power output curve (dashed power function 110), at which point the time rate of change of the power generated by the renewable power generator is equal to 0 (dP/dt=0). Therefore, methods of Maximum Power Point Tracking (MPPT) and maintaining the operation of a renewable power generator at or near the MPP are needed.
Prior art exists for methods of MPPT. The majority of prior art focus on taking a snapshot of the current and voltage output of the solar power generator at a time point, then calculating the output power, which is the product of the current and voltage, at that time point. This process repeats itself periodically, and the generated power calculated at a delayed point in time is compared to the generated power calculated at a time point that is non-delayed. If the power calculated at the non-delayed point in time is higher than the power calculated at the delayed point in time, the renewable power generator is operating to the left of the MPP or point 120 of the power output curve, denoted as dashed power function 110, and adjustment may be required to be performed in order to modify the operating point of the renewable power generator on the power output curve, until it coincides with the MPP located at point 120. If the power calculated at the non-delayed point in time is lower than the power calculated at the delayed point in time, the renewable power generator is in operation to the right side of the MPP or point 120 on the power output curve, denoted as dashed power function 110, and adjustment may be required to be performed in order to modify the operating point on the power output curve, until it coincides with the MPP.
The prior art discussed previously typically requires sophisticated circuits and digital intelligence, such as microprocessors and memory units to complete all necessary MPPT and output power adjustment operations. Furthermore, because of the non-continuous nature of digital electronics, it may be inefficient with respect to time for the operating point of the renewable power generator to converge to MPP.
An easier prior art approach, over the prior art approach discussed previously, does not utilize complicated, and potentially expensive digital components and takes advantage that, at the MPP on the output power curve, the time rate of change for the power generated by the renewable power generator is equal to 0 also denoted as dP/dt=0. One of the examples is given by U.S. Pat. No. 6,919,714, which presents a method of monitoring the direction of the time rate of change for the power generated. When the direction of the time rate of change is identified, the relative position of the MPP is known, and methods of making the renewable power generator operate at or near the MPP can therefore be implemented.
One of the more effective prior art methods for configuring a renewable power generator to operate at or near the MPP is to manipulate the load resistance so that it matches the internal resistance of the renewable power generator. It can be mathematically shown that a renewable power generator operates at MPP when its internal resistance equals to the load resistance. Therefore, some prior art implementations present methods of configuring the renewable power generator to operate at the MPP by adjusting the load resistance so that it continuously matches the internal resistance of the renewable power generator.
A prior art example illustrated in U.S. Pat. No. 4,494,180, presents a method of adjusting the output load resistance in order to match the internal resistance of a renewable power generator. For this case, the load is an AC electrical motor, which is driven by a DC/AC inverter powered by a renewable power generator. The resistance of the electrical motor is a function of the frequency of the AC power. As such, matching the resistance of the electrical motor and internal resistance of the renewable power generator is achieved by altering the frequency of the AC power, and the frequency of the AC power is adjusted according to the location on the output power curve where the renewable power generator is operating. This operating point is tracked by monitoring the direction of time rate of change for the power generated, denoted as dP/dt, by the renewable power generator. When the frequency of the AC power is adjusted to a value such that the direction for the time rate of change for the power generated is equal to 0, the resistance of the electrical motor is in a condition of matching the internal resistance of the renewable power generator, the renewable power generator operates at the MPP, and as a result the generation of power is maximized.
However, the prior art method previously mentioned is limited in that, only a specific load type, such as an AC electrical motor in the case of U.S. Pat. No. 4,494,180, can benefit from the method, and therefore more generic methods are needed to accommodate a diversity of load types. Furthermore, prior art implementations do not offer detailed implementations that specify methods of MPPT, and more importantly, methods of adjusting the load resistance based on the results of the MPPT in order to match the internal resistance of the renewable power generator.
In view of the foregoing, there is a need for improved techniques for enabling renewable power generators to operate at MPP and for improved techniques of MPPT.
Unless otherwise indicated illustrations in the figures are not necessarily drawn to scale.